I'm writing a game and map editor using Java and jMonkeyEngine. In the map editor, I've got a brush done by wireframed sphere. My problem is: I want to make it visible everywhere, so I want to invert color of the pixel "behind" this brush. I wanted to do it with fragment shader (GLSL), but I don't know how to get color of this pixel.
The quick and easy way to do this is to use the standard blending equation with the following parameters:
So you just need to set